44 MORPHOLOGY OF 



Among this loose tissue of the centre of the cord are remark- 

 able stellate groups of cells (fig. 78, stel.) wliose heads are thus 

 placed radiating from a small lumen, which is generally 

 sharply defined on three sides and usually irregularly bounded 

 at some part of its margin. The nature of these stellate 

 groups did not appear. They are commonest in the sides of 

 the " grey ■'■' tracts, viz. at the points where the white matter 

 is bent up {v. fig. 74, b). It is possible that the spaces thus 

 enclosed may in some indirect manner communicate with the 

 neural tube. 



The histology of the cord is nearly the same in all the 

 species. In B. salmoneus and B. Brooksii, however, there 

 is always a quantity of yellowish granules embedded in the 

 central substance (on the analogy of Nemertines this substance 

 may function like haemoglobin). The shape of the cord in 

 section varies in the difi'erent forms and in different parts of 

 its course {v. fig. 74). 



From the lower surface of the white matter of all species 

 many fibres may be seen leaving the cord and losing them- 

 selves among the subjacent muscular tissues. In B. Kowa- 

 levskii alone no connection exists between the dorsal side of 

 the cord and the skin. In B. minutus this is accomplished 

 by three cords of skin substance. Their outsides are covered 

 with a fibrous sheath (Spengel), and this is in connection with 

 the fibrous layer of the skin. As Spengel has stated, these 

 cords contain a more or less distinct lumen. I have not been 

 able to trace these out upon the skin, though they occasionally 

 appear to lead to the cavities enclosed by the radiating cells. 

 These cords I propose to term the dorsal roots. They occur 

 in B. minutus, Eobinii, salmoneus, and Brooksii. 

 Their homology will be discussed when the other morpho- 

 logical questions arising out of these facts are treated of. 



The histology of the rest of the nervous system has been 

 sufficiently described. 



The relations of the parts are explained by figs. 60, 64, 65, 

 67, 73, &c. 



There are no special sense organs. 



